I ve not much experience with a pet battle...I had two pvp fight after each other, and the opposite side was swapping my pets during the fight...How i remember it was 3-3 different pets.Is it a pet ability?

Feign Death is cool, it swaps your own pets (rather than the opponents) without the lost turn you get from manual switching (so your opponent does not get a free attack on you)... and the attack avoidance works even if the pet with the ability is the last one left alive on your team.